> Have to disagree. You take your pocket tape measure, carefully hook the end of the tape on the Sun and slowly walk to Pluto. You read what it says. That is a distance.

By that measure then, you carefully hook your tape measure to Fomalhaut and slowly walk out to the outer ring, whereupon your ancient eyes perceive the distance to be 120 AU, not 240 AU (since 240 AU is the diameter). A given rock in the outer ring is therefore less distant from Fomalhaut than Voyager is from the Sun.

That's all the parent is saying, and I agree with them - I, too, know the difference between radius and diameter but a quick read of your comment left me thinking that Fomalhaut's outer ring is orbiting further away from Fomalhaut than Voyager is from the Sun.

It's clearer to compare a radius to a radius, is all.